The present invention relates to power transmissions and more particularly to multi-speed power transmissions having six forward speed ratios.
Multi-speed power transmissions are incorporated into powertrains to provide an extended operating range for the internal combustion engine that supplies the power for the powertrain. The number of ratios that are incorporated into automatic transmissions, especially for passenger car and small truck usage, has increased from two forward speed ratios to five forward speed ratios. Currently some manufacturers are considering transmissions incorporating six forward speed ratios. Examples of such proposals can be seen in the United States Patents issued to Liu and Malloy (U.S. Pat. No. 5,046,999) on Sep. 10, 1991 and Haka (U.S. Pat. No. 5,577,976) on Nov. 26, 1996.
These transmissions generally employ a Ravigneaux gear arrangement comprised of a long pinion and two or more short pinions that are intermeshing. The pinions interconnect two sun gears and two ring gears. By combining at least six selectively operable torque transmitting mechanisms (clutches and brakes), six forward speed ratios and one reverse ratio is available. The gears of these planetary arrangements are aligned in two axial planes. However the overdrive ratios achieved with these arrangements have a large step and the sixth ratio has a low numerical value.
Other six speed transmissions, having three axial planes of gears, have been proposed. One such mechanism is shown in U.S. Pat. No. 4,070,927 issued to Polak on Jan. 1, 1978. This arrangement uses three simple interconnected planetary gear sets and five torque transmitting mechanisms. The transmission described in the Polak patent has been used extensively in large trucks.
It is an object of the present invention to provide an improved multi-speed power transmission having six forward ratios and one reverse ratio.
In one aspect of the present invention, the transmission has a compound gear arrangement including two sun gear members and two ring gear members that are interconnected by a plurality of intermeshing pinion gears laid out in three axial planes. In another aspect of the present invention, at least one of the intermeshing pinion gears is a stepped pinion gear. In yet another aspect of the present invention, one of the ring gear members is an output member and the other ring gear member is a reaction member during two of the forward ratios.
In still another aspect of the present invention, the reaction ring gear member is smaller in diameter than the output ring gear member. In a further aspect of the present invention, the reaction ring gear member is disposed in one axial plane, the output ring gear member is disposed in another axial plane. In yet a further aspect of the present invention the planes containing the ring gear members are separated by an axial plane containing one of the sun gear members. In still a further aspect of the present invention, six torque transmitting mechanisms are employed to selectively provide six forward ratios, including two overdrive ratios, and a reverse ratio wherein the highest overdrive ratio is equal to or greater than 0.50.